1. Field of the Invention
The present invention relates to a connection structure of optical fibers, specifically to a connection device that connects optical fibers having different core diameters and made of different materials, such as a plastic fiber and a glass fiber.
2. Description of the Related Art
In connecting optical fibers, the most important technical task is to achieve a low transmission loss of light as far as possible.
As an example, in connecting two optical fibers having the core diameter of 60 .mu.m and the specific refractive index difference of 0.7% with their end faces confronted with each other, it is generally conceived that an adequate transmission of light is difficult to be realized, unless, assuming that there is no crimp in the connecting area, the optical axis displacement is made within 0.2 .mu.m and the connection loss is made within 0.2 dB.
FIG. 17 illustrates a first conventional optical fiber connector 50 that has implemented such a low transmission loss. As illustrated, the first conventional optical fiber connector 50 includes a first optical fiber 51, a second optical fiber 52, a cylindrical connecting part 53 that connects the first and second optical fibers 51, 52, and a lens 54 contained in this connecting part 53, in which a light beam emitted from an end face 51a of the first optical fiber 51 falls on an end face of the second optical fiber 52 through the lens 54. Thus, the two optical fibers are optically connected.
FIG. 18 illustrates a second conventional optical fiber connector 60, which includes a first cylindrical connecting part 65 with a flange 65a and a second cylindrical connecting part 66 with a flange 66a, and further a first lens 63 contained in the first connecting part 65 and a second lens 64 contained in the second connecting part 66.
First and second fixing parts 67, 68 having holes on the centers thereof, are mounted on the ends of the opposite sides to the flanges 65a, 66a of the first and second connecting parts 65, 66. Opposing ends of the first optical fiber 61 and the second optical fiber 62 are guided in the center holes of the fixing parts 67, 68.
The first and second lenses 63, 64 are fixed inside the first and second connecting parts 65, 66, respectively, so that the optical axes coincide with each other; and thereafter, the first and second connecting parts 65, 66 are attached so that the flanges 65a, 66a are engaged with each other. The first and second fixing parts 67, 68 are fastened to the first and second connecting parts 65, 66 with screws, etc. The first and second optical fibers 61, 62 are stripped of the sheathing parts from the front ends thereof, and the stripped ends each are engaged in the center holes of the fixing parts 67, 68.
Thus, the first and second optical fibers 61, 62 are configured so as to form the focuses on the end faces thereof, for example, a light beam emitted from the end face of the first optical fiber 61 is emitted as a parallel beam from the first lens 63, and the parallel beam falls on the second lens 64, which transforms the beam into a convergent beam. Thus, the two optical fibers are optically connected.
FIG. 19 illustrates a third conventional optical fiber connector 70, which includes a first optical fiber 71, a second optical fiber 72, two glass spheres 75 in contact with each other. The two glass spheres 75 are disposed between both front ends of the first and second optical fibers 71, 72. The connector 70 also includes, liquid substances 78 having a refractive index of approximately 1, which are inserted between the first optical fiber 71 and one glass sphere 75 and between the second optical fiber 72 and the other glass sphere 75, and a molded resin 80 that sheathes an area including both the front ends of the first and second optical fibers 71, 72, the liquid substances 78, and the two glass spheres 75 thus disposed.
A parallel beam emitted from the first optical fiber 71 falls on the one glass sphere 75 and converges at a point 76 where the two glass spheres come into contact. The convergent beam is transformed into a parallel beam through the other sphere 75, which falls on the second optical fiber 72. Thus, the two optical fibers are optically connected with a symmetrical optical path.
However, in such optical fiber connectors 50, 60, 70, a high positioning accuracy in the connection of the two optical fibers is required in order to transmit a stable light beam through the optical fibers connected. Further, it is necessary to enhance the efficiency of optical connection through the optical elements such as the lenses, so that a lower transmission loss of light than that obtained by the optical fibers being directly connected can be achieved.
Furthermore, in the connection of the two optical fibers having different diameters, for example, in the connection of a plastic optical fiber (POF) and a fused quartz fiber (PCF) having different core diameters, a higher positioning accuracy is required than the connection of optical fibers having the same diameter, and a low transmission loss of light has been difficult to be realized.